| Issue |
146642
|
| Summary |
[InstCombine] Missed optimization for icmp of select with constants and invertible binop
|
| Labels |
llvm:instcombine,
missed-optimization
|
| Assignees |
|
| Reporter |
nikic
|
We currently fail to fold variations of this pattern:
https://alive2.llvm.org/ce/z/tT3yNU
```llvm
define i1 @src(i64 %a, i1 %c) {
%idx = shl nsw i64 %a, 3
%sel = select i1 %c, i64 8, i64 0
%cmp = icmp eq i64 %idx, %sel
ret i1 %cmp
}
define i1 @tgt(i64 %a, i1 %c) {
%sel = select i1 %c, i64 1, i64 0
%cmp = icmp eq i64 %a, %sel
ret i1 %cmp
}
```
The underlying pattern here is something like `f(x) == y` converted to `x == f^-1(y)` where f invertible and `f^-1(y)` folds because the shift arms are constant.
>From https://github.com/dtcxzyw/llvm-opt-benchmark/pull/2524/files/9ca17b717ceea34b52263bf8b20cf12ad02cd4fd#r2179220735.
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